1. Field of Invention
The invention relates to tube joints. It concerns in particular joints for tubes having a small diameter and/or a thin wall, such as are used for distributing fuel in aircraft or automobile engines.
2. Discussion of Prior Art
Generally, for the purpose of forming such a joint, the outer end of the tube to be joined is first provided with a cylindrical sleeve which can be anchored on said end, after which the sleeve is pressed into a joint member with a nut which is screwed on to the latter. It is essential that the anchoring of the sleeve on the tube should not cause the formation of lines or zones or reduced strength, particularly if the tube is of small wall-thickness. The process that seems to have met these technical anchoring requirements the most satisfactorily comprises the expansion of the end of the tube within the sleeve. The expansion process is very old and consists in introducing, into the tube, a mandrel of variable diameter so as to cause radial internal pressure which urges the tube against the inner wall of the sleeve which is provided with ribs and channels. In this method, an axially compressible mandrel is introduced into the tube, and then, with the help of a suitable system, the circumference of the mandrel is caused to expand in the zone of the channels in the sleeve, and the wall of the tube is thus radially pressed towards the bottoms of said channels. Then, after this interior shaping of the tube has been completed, the compression force is relaxed and the mandrel is withdrawn. The tube remains anchored in its sleeve and can then be connected in the usual manner by means of a joint member and a nut which can be screwed on to it.
The sleeves or sockets most frequently used are relatively long and for the purpose of achieving adequate anchoring, they comprise two spaced channels defining a separating rib and an end rib. Since the extent to which the tube can be deformed is limited, the presence of a plurality of ribs in the sleeve is necessary to ensure an adequate latching action.
Although this process has been generally satisfactory, it involves difficulties and has been found inadequate for reliably meeting technical requirements that are becoming progressively more stringent. In the first place, the compressible mandrel is fragile and breaks when used in tubes of small diameter, and the process is increasingly more difficult to use as the wall-thickness of the tube diminishes. In the zone of the ribs and channels of the sleeve, and consequently in the zone where the diameter is expanded, the tube is stretched radially and longitudinally, and the thickness of its wall therefore diminishes slightly, and this is noticeable in a tube having a very thin wall. The presence of the ribs causes folding of the material of the tube accompanied by the onset of lines of reduced strength which are aggravated when the sleeve is pressed into the joint member and into the clamping nut. Furthermore, due to the presence of two channels, the conventional excess thickness of a sleeve corresponds only to the central channel, and a thin zone corresponds to the end channel, and this zone is weak.